People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

Saliva Can Help Detect Cancer

Saliva could be used to screen a person for diseases, even certain cancers, just like blood, say scientists from University of California, Los Angeles.

Dr. David Wong from the UCLA School of Dentistry writes in an article that human saliva has proteins, DNA, RNA that make it a perfect tool for assessing a person's heath. Saliva can be used to detect diseases of the mouth and oral cancers, but whether saliva has biomarkers that can help detect presence of other non-oral cancers wasn't known.

Wong says that new research of molecules present in the saliva has shown that it has biomarker for a type of early stage pancreatic cancer.

Salivanomics (the study of the biological molecules in saliva) researchers say that the saliva can potentially be used to detect cancers, diabetes, autoimmune diseases, etc., and because getting a saliva sample is relatively easier than drawing out blood, the screening could be faster.

The saliva-based diagnostics could work well in the U.S., the researchers say, because 20 percent Americans visit their dentists more often than their physicians. Dentists could act as a primary healthcare provider, checking the saliva to see if the person has any disease.

Wong says that as of March 2012, no test has been developed that can be used to detect a disease using saliva. The FDA approved OraQuick ADVANCE Rapid HIV-1/2 Antibody Test screen for antibodies that could show an HIV infection but the test isn't accurate enough to prove an HIV infection.